Proposed extension to HTML5.1 for rollup captions

Hi all,

This is a proposal to extend the TextTrack API in HTML with new objects
that will enable the display of captions/subtitles as scrolling text, and
introduce the ability to move a group of caption/subtitle cues from one
location on the video viewport to another. The feature will allow
presenting rollup captions from all text track formats that a browser
supports, including WebVTT and TTML.

Specification:
https://dvcs.w3.org/hg/text-tracks/raw-file/default/608toVTT/region.html#extension-of-the-html-texttrack-api
(Only the Appendix is relevant to the HTML WG).

I'm looking for feedback on this feature here. If there are no objections
and sufficient support for these features, I suggest adding them to the
HTML5.1 specification.


The use case:
============
Summary: Video users of diverse nations are used to the use of rollup
captions from TV. Deaf users have asked to have all TV content be displayed
on the Web in the same format. In the US it is a legal requirement to
support rollup captions on all video devices.

Details:
The display of captions/subtitles as scrolling text is known as "rollup
captions" (e.g. [0]). It is one of three standard display styles for
captions on TV [1]. It is often used when material is live captioned and
for post-production broadcast programming that only has one speaker. [2]

Rollup is part of the Consumer Electronics Association specifications of
captioning for analog TV (CEA608) and for digital TV (CEA708). It is not
only supported on TV but also on DVD through the SCC format [4]. It is used
in the US, Canada [5], Australia, and probably other countries. The
QuickTime file format supports it (e.g. [6]).

Rollup captions on all video devices, including devices that display Web
content, are a legal requirement in the US, introduced through the
Communications and Video Accessibility Act (CVAA) by the Federal
Communications Commission (FCC) 79.103 [7].


The proposal:
===========
Summary: Introduce a TextTrackRegion object that is a video viewport region
and can hold multiple TextTrackCues. The properties of TextTrackRegion
specify the display location and text scrolling mechanism for the cues it
contains.

Details:
In the TextTracks Community Group we have analysed how to introduce rollup
captions using WebVTT and the HTML <track> element.

11 different proposals were analysed:
http://www.w3.org/community/texttracks/wiki/RollupCaptions .

One proposals was picked and developed further:
http://www.w3.org/community/texttracks/wiki/MultiCueBox
There is a trial implementation in JavaScript for WebVTT:
https://github.com/silviapfeiffer/WebVTT-with-regions
Example rendering:
http://html5videoguide.net/test/WebVTT-with-regions/player.html (needs a
browser that supports TextTrack, so don't use Firefox).

An experimental implementation in WebKit is forthcoming:
https://bugs.webkit.org/show_bug.cgi?id=109570

Note: Ian Hickson is not supportive of this feature in WebVTT:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=20349 and recommends
browsers to implement SCC [4] support instead. However, the objects
proposed in this extensions specification would still be required to
represent the feature.

Are there any objections towards introducing these extensions into HTML5.1?

[0] http://www.slideshare.net/prithviraj_karbar/roll-up-captions-sample
[1] http://www.dcmp.org/caai/nadh38.pdf
[2]
http://abercap.com/blog/2009/03/17/understanding-the-differences-between-roll-up-captioning-pop-on-captioning-and-subtitling/
[3] http://www.atsc.org/cms/standards/a53/a_53-Part-3-2009.pdf
[4] http://www.theneitherworld.com/mcpoodle/SCC_TOOLS/DOCS/SCC_FORMAT.HTML
[5] http://www.cab-acr.ca/english/social/captioning/captioning.pdf
[6] http://zeitanker.com/common/movies/PedPoster.mov
[7] http://www.hallikainen.com/FccRules/2012/79/102/index.php

Best Regards,
Silvia.